1. Field of the Invention
The present invention relates to an optical transmission path monitoring system, a monitoring apparatus and a monitoring method therefor, and more particularly to a formula of optical transmission path monitoring for monitoring the states of optical fibers and optical amplifier-repeaters constituting optical transmission paths in a wavelength division multiplexing optical transmission system by utilizing an optical time domain reflectometer (OTDR).
2. Description of the Related Prior Art
Today, as a technique of large-capacity long-distance communication, wavelength division multiplexing transmission (WDM transmission) systems using optical amplifier-repeaters have come into practical use extensively. In such a system in actual operation, if any fault arises on an optical transmission path, pinpointing to identify the position and cause of the fault is extremely important for early restoration of the system. The OTDR method by which the intensity and position of reflection of reflected light from midway on the optical transmission path are measured by using an optical pulse is one of the major techniques available for use in fault pinpointing on an optical transmission path.
The ITU-T G. 977 recommendation also refers to the use of a coherent OTDR (COTDR) for use in fault pinpointing in a long-distance optical fiber system. In addition to this recommendation, a number of proposals have been made regarding the monitoring of long-distance WDM optical transmission paths using an OTDR. They include, for instance, the Japanese Patent Applications Laid-Open Nos. 1996-181656, 1999-266205, 2000-31907 and 2000-59306. All the techniques disclosed therein are intended to monitor optical amplifier-repeaters. According to any of these techniques, a transmitting station transmits a monitoring light differing in wavelength from a signal light over a transmission path; the monitoring light is looped back midway on the transmission path to another route in the reverse direction; and the transmitting station simultaneously monitors a plurality of optical amplifier-repeaters by receiving the looped-back monitoring light.
Generally, a long-distance optical transmission path has a configuration in which optical fibers and optical amplifier-repeaters are connected alternately in cascade and in multiple stages. On account of the long distance, it is extremely important for restoration from the faulty state to accurately identify the position and cause of the fault. On the other hand, the optical fibers and the optical amplifier-repeaters, which constitute the optical transmission path, differ in light transmission characteristics. For this reason, in order to monitor an optical transmission path and identify the position and cause of any fault that may arise, the two types of constituent elements, optical fibers and optical amplifier-repeaters, should be differentiated from each other and efficiently monitored without having to take much trouble. Any of the above-cited examples of the prior art merely proposes optical transmission path monitoring by which either only optical amplifiers are mainly monitored or optical fibers and optical amplifier-repeaters are collectively monitored as an optical transmission path without strictly differentiating them.